The field of therapeutic ultrasound is emerging with strong potential and broad medical applications. Characterized by its ability to penetrate at depth inside the body without harming intervening tissue, ultrasound has posed the basis for a new array of noninvasive therapies. In particular, great effort is concentrated in the field of High Intensity Focused Ultrasound (HIFU) to provide a noninvasive means for systematic tumor treatment, acoustic hemostasis, and targeted drug delivery. At a recent meeting of the International Society for Therapeutic Ultrasound held in Seattle, WA in June, 2003, the Chinese reported swift progress, with 3 companies selling government-approved devices, with >75 devices in use, and with ~7,000 patients having been treated for a variety of benign and malignant tumors. However, it was also clearly discernable that general acceptance of this technology by clinicians in the USA, and probably much of Europe, was still relatively distant. A major reason for this lack of acceptance outside of China is that real-time monitoring and control of the therapy is still a major challenge. One step toward a solution of this problem is to develop an accurate model for HIFU therapy. The obvious benefits of such a model are as follows: (1) It will enable us to design acoustic delivery systems before incurring the expenses of construction; (2) it will enable us to plan experiments to maximum efficiency; (3) it will reduce the number of animals used; (4) it will reduce the time between the developmental phase and clinical trials; and (5) it will enable us to optimize treatment planning to achieve maximum results in the minimum amount of time. The major goal of this proposed research is to develop a new numerical approach for comprehensive and fast HIFU simulation software based upon scalar and elastic representations of ultrasound wave propagation, and to validate the model with a series of ex vivo and in vivo experiments. The new model will account for realistic physical processes and can be applied consistently in every region of the human bodyindependently of media characteristics and geometries. The development of this model will support on-going studies in our laboratory, and others, of new engineering designs for HIFU applications.